Influence of Laser-Assisted Fusing on Microstructural Evolution and Tribological Properties of NiWCrSiB Coating

Abstract

The present study examines the applicability of a diode laser-assisted fusing treatment and a temperature-control system to the NiWCrSiB thermal spray coating to develop the enhanced wear resistance of continuous-casting molds. As a result of the use of the lasers, the variations in the microstructure and the hardening behavior during the fusing treatment could be controlled. Fine secondary phases (approximately 0.05–10 μm in size) homogeneously present in the coating after the laser-assisted fusing were observed to be Cr-, Mo- and W-based carbides and borides. Transmission electron microscope analysis was used to characterize these fine secondary phases as M7C3 and M23C6 carbides and M5B3 boride. Because of these fine secondary phases, the hardness increased from 730 (as-sprayed status) to 1230 HV (after fusing at a temperature of 1473 K). Finally, given the formation of fine secondary phases and the occurrence of surface hardening, the laser-assisted fusing treatment was deemed to enhance the tribological performance of the thermal-sprayed coating, in that it exhibited a lower coefficient of friction and lower wear rate than the as-sprayed coating.